Bearings are lubricated by oil or grease, for example. The lubricant is stored in a reservoir and is from there transported to the bearing. The used lubricant leaves the bearing.
The lubricant needs to be pure enough to avoid damages in the bearing by impurities or particles.
Impurities, like particles of different kind, can cause severe damage to the bearing. The live-time of the bearing is reduced or the bearing is damaged in a way that it can no longer be used and needs to be replaced.
Depending on the type of bearing, and the use of the bearing, the costs of the exchange of a bearing can be extremely high. For example, in the case of a bearing used as a main bearing in an offshore wind turbine, the exchange of the bearing requires the use of a ship and a crane to detach the rotor of the wind turbine and replace the bearing.
Thus damages to the bearing, and thus an early exchange of the bearing, need to be avoided. Therefore impurities, like for examples particles in the lubricant for the bearings need to be avoided.
Particles are sometimes present in the lubricant as it is delivered read to be used. In addition, on the way from the reservoir to the bearing, the lubricant might get impure by dust and particles from the pump or the conduits.
It is known to filter oil, to remove impurities and particles. Oil can easily be forced through a filter, as its viscosity is low enough to allow filtering. The viscosity of grease is higher which makes filtering of grease difficult.